1. Technical Field
The present disclosure relates to an audio-output amplifier circuit for an audio device, an audio device, an electronic device including the audio device, and an output control method to switch the audio device, and more particularly to an audio-output amplifier circuit to reduce pop more stably.
2. Description of the Related Art
It is well known that audio devices sometimes generate an unusual crackling sound (pop) when a power supply of an audio output amplifier of the audio device is turned on or off. As a technology for reducing pop, JP-2011-029683-A discloses a pop reduction circuit to control an audio-output amplifier circuit so that an output voltage of an output amplifier of the audio-output amplifier circuit transits smoothly, using raised cosine and ramp waveforms, when the output amplifier is switched on and off.
FIG. 1 is a circuit diagram illustrating a configuration of an audio-output amplifier circuit 7X including an output amplifier 1. FIG. 2 is a timing chart illustrating operation of the audio-output amplifier circuit 7X shown FIG. 1, assuming that a time-constant determined by a resistance of an output resistance load and a capacitance of a DC-cut capacitor is large.
In this example shown in FIG. 1, a capacitor 3 (DC-cut capacitor) is attached to an audio device 8X including a pop reduction circuit 6, and the DC-cut capacitor 3 blocks a direct-current (DC) component of the output signal when the audio output amplifier 1 of the audio device 8X is turned off. With this configuration, when a time-constant determined by a resistance RL of an external resistance load 4 and a capacitance C1 of the DC-cut capacitor 3 is large relative to a transition time of an output voltage VOUT, a residual charge is remaining in the DC-cut capacitor 3 even after the transition of the output voltage VOUT is finished.
Therefore, if the output voltage VOUT of the audio device 8X is turned off in this condition, a pathway to discharge the residual charge is shut off, and a level difference may occur in the waveform (11, 12 shown in FIG. 2) after the capacitor 3 blocks the DC because an output voltage VOUTRL after the DC cut suddenly rises, which may generate the pop.
A specific example is described below with reference to FIGS. 1 and 2. In FIG. 1, the output amplifier 1 is switched on and off based on an audio output control signal PD, and an output terminal (amplifier-output terminal) 1a of the output amplifier 1 is connected to an output terminal 2 of the audio device 8X. The output terminal 2 is connected to ground via the DC-cut capacitor 3 and the resistance load 4 A pop reduction circuit 6 that generates a reference voltage VREF input to the output amplifier 1 is provided in a previous stage of the output amplifier 1.
In the audio-output amplifier circuit 7X of this example, immediately before the output amplifier 1 is switched off, the pop reduction circuit 6 varies the reference voltage VREF so that the output voltage VOUT transits from a setting reference voltage value VSET to a ground-level voltage 0. Then, the output amplifier 1 is switched off after the output voltage VOUT at the outer terminal 2 reaches the ground-level voltage 0. When the output amplifier 1 is switched off, the amplifier-output terminal 1a enters a high-impedance state.
In FIG. 2, a primary delay element, based on the time-constant determined by the resistance RL of the external resistance load (output load) 4 and the capacitance C1 of the DC-cut capacitor 3, dominantly acts on the output voltage VOUTRL of the output load 4 after the DC-cut capacitor 3 blocks the DC component, and accordingly, transition of the waveform of the output voltage VOUTRL across the external load 4 is delayed compared with the waveform of the output voltage VOUT at the output terminal 2.
Then, even after the transition of the output voltage VOUT is finished and the output voltage VOUT reaches the ground-level voltage, the output voltage VOUTRL, after the DC-cut capacitor 3 blocks the DC, does not reach the ground-level voltage because of the charge remains in the capacitance C1 of the DC-cut capacitor 3.
In this state, if the output amplifier 1 is switched off and the amplifier-output terminal 1a of the output amplifier 1 rises to a high impedance, a pathway to discharge the charge remaining in the capacitance C1 of the DC-cut capacitor 3 is shut off, which generates a large difference (step) in level of the waveform of the output voltage (11, 12 shown in FIG. 2). Therefore, the pop may be generated.